1. Field of the Invention
The invention concerns an improved cross-laminate of oriented films of the general type defined in the preamble of claim 1 and improved processes and apparatus for manufacturing cross-laminates of this general type.
2. Description of Related Art
Processes for production cross-laminates of oriented films are known in particular from GB-A-1,526,722. In this known invention, the material is melt oriented in a generally uniaxial manner before cross lamination, and is biaxially oriented after cross lamination, preferably near room temperature. The melt orientation may be very weak, but is always combined with the use of blends of polymers, which are sufficiently incompatible for formation of a two- or multiphase grain of polymer under the influence of the melt-orientation, which grain has a very significant influence on the strength properties of the final cross-laminate. In order to enhance the tear-propagation resistance the bonding between the films is a generally weak bonding, but may be supplemented with a strong bonding in spots or lines.
The objective of this known invention is to provide for a film material which exhibits high strength properties in all respects. One of its important uses is for bags and similar packaging uses.
Further according to the above mentioned British Patent, the transverse stretching which is subsequent to the cross-sandwiching of the melt-oriented films (see the introduction to claims 12 and 24) is preferably carried out by passing the sandwich through several sets of mutually intermeshing grooved rollers, the grooves of which are as fine as practically possible. Also, the sandwich is normally (but not necessarily) stretched longitudinally in continuous manner between smooth rollers before, between or after said steps of grooved rollers stretching. (I distinguish between "sandwiching" which may involve, but needs not involve a bonding of the films to one another, and "laminating" which always involves such bonding). For obtaining optimum energy-absorbing properties (such as e.g. shock-tear propagation resistance) the different stretching steps, following the cross-sandwiching, are preferably carried out at temperatures very much below the melting ranges of the films, and may even be carried out at normal room temperature.
Further according to the above mentioned British Patent, the cross-lamination of films having a uniaxial or an unbalanced biaxial melt-orientation can be carried out already in the extrusion process under use of counterrotating dieparts, but can also be established on the basis of helical cutting of melt-oriented, tubular films. Thus the tubular films can be melt-oriented mainly in their longitudinal direction, helically cut e.g. under 45 degrees after solidification, and subsequently sandwiched in such manner that the said main directions criss-cross one another (i.e. become perpendicular to one another if the cutting angles all have been 45 degr.). In this connection, recent WIPO publication WO-A-89/12533 discloses particularly practical methods for spiral cutting of tubular film, and also discloses a suitable method to achieve a melt-orientation, which, if desired, can be perpendicular to the machine direction (i.e. the continous direction) of the film. The last mentioned method consists in first hauling-off the tubular film from the extrusion die in a screw-movement to give the tubular film a melt-orientation which forms an angle (e.g. 30 degrees) to the axis of the tube, and then helically cutting the tubular film (e.g. under 60 degrees) in the way which increases the angle between the machine direction and main direction of melt-orientation. Thus, using the above mentioned example that the "screwing" is under 30 degr. and the cutting under 60 degr., the melt orientation will become perpendicular to the machine direction after the helical cutting. This film can continuously be sandwiched with a film which is melt-oriented mainly in its longitudinal direction (the machine direction) to form a perpendicular criss-cross arrangement.
One of the features mentioned in the introduction to the present claim 1 is that the cross-laminate exhibits a pattern of striations constituted by thickness variations. Such pattern will always be formed as a result of the stretching between grooved rollers--except if special precautions are taken, which will be discussed later. Following the teaching of the above mentioned British Patent (see in particular FIGS. 8 and 9) these thickness variations which form a longitudinally striated pattern, will occur at random as a result of interference between the stretching patterns in each step of the grooved roller stretching. When not exaggerated, the striated pattern can have a positive influence on the tear-propagation properties and some positive influence on the self-supporting capability (the stiffness when the material is bent over a line perpendicular to its continuous direction). However, if this random pattern of thickness variations is very pronounced, it has a very negative influence on u-v stability, printability and the resistance to penetration by humidity, aroma substances and vapours.
As a special feature of the technology, the individual films can be blocked together by the transverse stretching together between grooved rollers, and this effect can be controlled by suitable surface layers on the films, which therefore originally are produced by coextrusion. In the coextrusion process, there is also made provisions to give the final laminate thin surface layers of desirable properties, in particular layers to improve heat-sealing or layers to control the frictional properties.
Further developments of the known technology referred to above are disclosed in U.S. Pat. No. 4,629,525. This describes a stabilization process, in which a cross-laminate of the above mentioned kind is heated while allowing at least 7% transverse shrinkage (i.e. transverse of the continuous direction of the laminate and of the striations produced by the grooved rollers) and preferably also a longitudinal shrinkage. Besides the stabilizing effect, which means that the coldstretched laminate does not tend to shrink further during use or storage at normal temperatures, there are important side-effects. One is that the above mentioned thickness variations (the striation effect) can become significantly reduced, since the transverse shrinkage mainly takes place where the material is transversely over-stretched. Another important by-effect is a significant increase of the yield point in the transverse direction. A third effect is increase of the weak bonding which originally is produced by the blocking-together of the spiral cut films between the grooved rollers.
The transverse contraction is preferably obtained by feeding the cross-laminate, while it is pleated to a suitable extend onto a heated roller (from where it may continue over more heated rollers) so that the pleating gradually disappears while the cross-laminate contracts.
This patent also discloses advantageous polymer blends for the main layer of the coextruded film for this general type of cross-laminates, in particular blends of high-molecular-weight-high density polyethylene and linear-low density or linear-low-density-like polyethylene at significantly lower molecular weight than the first mentioned component, to which optionally may be added polypropylene (as these cross-laminates are more precisely defined in claims 25-29 of the above mentioned patent). Finally, the said patent specification discloses that cross laminates for manufacture of sacks preferably should be made from melt-oriented tubular film cut under an angle between 10.degree. and 35.degree. instead of 45.degree.. Further improvements in the general type of cross-lamination technology described above is disclosed in WO-A-88/05378. Here, at least the first pair of grooved rollers is of special construction and function. The grooved, fine, circular "teeth" have inclined sidewalls, the sidewalls on the cooperating grooved rollers match very exactly, and they operate under a high roller's pressure so that the transverse stretching takes place not only by tentering but also by squeezing or "lateral calendering" of the laminate or sandwich (all as further described in that citation).
By this method it has been possible to manufacture above described cross-laminates in improved quality and at highly increased production capacity. This increase is made possible because two or more cross laminates can be produced together in this process, and separated from one another at the end of the manufacturing process.
The inventor has also combined into the technology the embossment localised adjacent to heat-seals in a bag, which is described in WO-A-89/10312 and which is adapted to produce a shock-absorbing or force-controlling effect thereby improving the drop strength of a heat-sealed bag of oriented or rigid film material and filled with powder or granulated goods.
By a combination of the above mentioned inventions the inventor has been able to manufacture in a commercial and economically feasible process, heat-sealed heavy-duty bags of cross-laminates in gauge e.g. 60-80 gm.sup.-2, which with respect to yield point tensile strength, puncture resistance, tear propagation resistance and drop-performance have proven superior to bags from low-density or linear-low-density polyethylene of the double gauge. However, due to lack of self-supporting capability of the cross-laminate in such gauges ("flimsiness") the bags have not yet met general market acceptance, since the automatic or manual handling in connection with the filling (the "bagging") has been considered too difficult or unreliable. In this connection it is noted that the self-supporting capability (which is a result of the stiffness of the film) of a film of even thickness varies with the second power of its thickness.